package/mender: set version in go linker

This is used when calling the mender client with the
-version option and it says "unknown" if not set in
linker.

Now it displays the following:

    # mender -version
    1.4.0
    runtime: go1.10.2

Signed-off-by: Mirza Krak <mirza.krak@northern.tech>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/mender/mender.mk

@@ -32,6 +32,8 @@ MENDER_LICENSE_FILES = \
 	vendor/github.com/ungerik/go-sysfs/LICENSE \
 	vendor/github.com/bmatsuo/lmdb-go/LICENSE.mdb.md
 
+MENDER_LDFLAGS = -X main.Version=$(MENDER_VERSION)
+
 define MENDER_INSTALL_CONFIG_FILES
 	$(INSTALL) -d -m 755 $(TARGET_DIR)/etc/mender/scripts
 	echo -n "2" > $(TARGET_DIR)/etc/mender/scripts/version